1. Field of the Invention
The present invention relates generally to compositions capable of absorbing ultraviolet radiation, their preparation, and their use as sunscreens. More particularly, the invention relates to synthetic melanin aggregates in the form of a melanin pigment incorporated in a polymer particle matrix.
With growing concern over the deleterious effects of sunlight on human skin, a number of ultraviolet absorbent substances have been identified and developed for use in topical sunscreen compositions. In general, the substances function by absorbing radiation in the ultraviolet radiation region and reemitting the absorbed energy in other, less harmful radiation spectrums.
Although generally effective, most of the sunscreen substances which are presently in widespread use suffer from a number of drawbacks. In particular, most of the sunscreen substances are very oily and are difficult to formulate into suitable products. Such sunscreen substances may also be incompatible with other ingredients in a cosmetic preparation where it would be desirable to incorporate sunscreen protection. Additionally, many sunscreen compositions are characterized by strong odor, usually requiring the addition of a fragrance in order to formulate a commercially acceptable product. Furthermore, the sunscreens are easily lost from the skin either by rubbing or by intradermal penetration. Finally, many of the widely used sunscreen substances can be toxic at high concentrations when absorbed through the skin.
For these reasons, it would be desirable to utilize alternate ultraviolet absorbent substances as the active ingredient in a wide variety of sunscreen and cosmetic formulations. In particular, it would be desirable to identify and provide ultraviolet absorbent substances which are non-penetrating and will not be rapidly lost from the skin. It would be further desirable if such sunscreen substances were non-toxic, even if applied to the skin over open wounds. Such sunscreens should be relatively non-odoriferous, and compatible with a wide variety of sunscreen and cosmetic preparations.
Melanins are a major class of broad-spectrum ultraviolet-absorbing organic polymers found naturally in the human epidermis. They are formed in the epidermis by the enzymatic conversion of L-tyrosine into L-3,4-dihydroxyphenylalanine, commonly referred to as L-dopa. The L-dopa is further converted to a melanin by a biologic pathway which is described in the literature. The primary class of melanins produced in the human epidermis is eumelanins, characterized by a black-brown color and composed primarily of tyrosine-derived units. Pheomelanins constitute a second class of melanins, characterized by a reddish-brown color and composed of cysteine-derived units in addition to tyrosine-derived units.
The use of melanins as the ultraviolet-absorbent substance in sunscreen compositions would have a number of the desirable properties set forth above. They are natural substances and substantially non-toxic even when applied at very high concentrations on the skin. They are free from objectionable odor and appear to be compatible with a wide variety of formulations, including both primary sunscreen formulations and cosmetic formulations. Finally, melanins are not volatile and will not be lost from the skin through evaporation or penetration.
Unfortunately, melanins incorporated in conventional carriers and vehicles, such as lotions, creams, ointments, powders, and the like, have not been found to provide effective protection against exposure to ultraviolet radiation when incorporated in primary sunscreen products or cosmetic formulations.
For that reason, it would be desirable to provide a suitable carrier or vehicle for incorporation of melanin pigments which, when applied topically to the skin, will afford a high level of protection against ultraviolet radiation. It would be particularly desirable if such compositions retained the other advantages which would be expected of melanins, i.e., non-toxicity, compatibility with a wide variety of formulations, freedom from odor, and the like.
2. Description of the Background Art
U.S. Pat. No. 3,698,852, describes a hair dye preparation prepared by the reaction, under alkaline conditions, of at least one dihydroxyphenylalanine or dihydroxyphenylglycine with a phenylamine. U.S. Pat. No. 4,515,773 describes an artificial tanning composition including a suitable dye precursor, such as tyrosine, L-dopa, or the like, in combination with tyrosinase. The tyrosinase catalyzes the conversion of the precursor into melanin-like dyes on the human skin to impart a tan-like appearance.